5G communications systems use a higher carrier frequency (referred to as high frequency) than a Long Term Evolution (LTE) system. Generally a 6 GHz frequency or above may be referred to as a high frequency. Frequency bands such as 28 GHz, 38 GHz, and 72 GHz are currently researched as a focus, to implement wireless communication with a larger bandwidth and a higher transmission rate. However, a high-frequency system have a more serious intermediate radio frequency distortion, especially stronger phase noise impact relative to conventional low-frequency communication. In addition, the impact of a Doppler shift and a carrier frequency offset (CFO) may increase as a frequency increases.
Multiple-input multiple-output-orthogonal frequency division multiplexing (MIMO-OFDM) is used as an example. In consideration of phase noise and carrier frequency offsets at both a receive end and a transmit end, a receiving expression for an nth receive antenna on a kth subcarrier after fast Fourier transform (FFT) at the receive end is:
            Y      n      k        =                            ∑                      m            =            1                    M                ⁢                              H                          n              ⁢                                                          ⁢              m                        k                    ⁢                                                    P                n                                  r                  ,                  0                                            ⁢                              P                m                                  t                  ,                  0                                                                    ︸              CPE                                ⁢                      S            m            k                              +                                    ∑                          m              =              1                        M                    ⁢                                    ∑                              i                =                0                                            K                -                1                                      ⁢                                          P                n                                  r                  ,                                                            (                                              k                        -                        i                                            )                                        ⁢                    K                                                              ⁢                              H                                  n                  ⁢                                                                          ⁢                  m                                i                            ⁢                                                ∑                                                            l                      =                      0                                        ,                                                                                                                        l                            ≠                            i                                                    &                                                ⁢                        l                                            ≠                      k                                                                            K                    -                    1                                                  ⁢                                                      P                    m                                          t                      ,                                              (                                                  i                          -                          l                                                )                                                                              ⁢                                      S                    m                    l                                                                                                ︸          ICI                    +              Z        n        k              where                    P        n                  r          ,          k                    =                        1          K                ⁢                              ∑                          t              =              0                                      K              -              1                                ⁢                                    e                              j                ⁢                                                                  ⁢                                                      θ                    n                    r                                    ⁡                                      (                    t                    )                                                                        ⁢                          e                              j                ⁢                                                                  ⁢                2                ⁢                π                ⁢                                                                  ⁢                                  tk                  /                  K                                                                          ,                            P          m                      t            ,            k                          =                              1            K                    ⁢                                    ∑                              t                =                0                                            K                -                1                                      ⁢                                          e                                                                                          ⁢                                      j                    ⁢                                                                                  ⁢                                                                  θ                        m                        t                                            ⁡                                              (                        t                        )                                                                                                        ⁢                              e                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      tk                    /                    K                                                                                          ;      and                  in      ⁢                          ⁢      this      ⁢                          ⁢      case        ,                  ⁢                  P        n                  r          ,          0                    =                        1          K                ⁢                              ∑                          t              =              0                                      K              -              1                                ⁢                      e                          j              ⁢                                                          ⁢                                                θ                  n                  r                                ⁡                                  (                  t                  )                                                                          ,                  P        m                  t          ,          0                    =                        1          K                ⁢                              ∑                          t              =              0                                      K              -              1                                ⁢                                    e                              j                ⁢                                                                  ⁢                                                      θ                    m                    t                                    ⁡                                      (                    t                    )                                                                        .                              
Hnmk indicates a channel from an mth transmit antenna to the nth receive antenna on the kth subcarrier, Smk indicates sent data of the mth antenna on the kth subcarrier, Znk indicates noise on the nth receive antenna on the kth subcarrier, Pnr,k indicates a phase offset on the nth receive antenna on the kth subcarrier that is caused by the phase noise and the CFO at the receive end, and Pmt,k indicates a phase offset on an mth transmit antenna on the kth subcarrier that is caused by the phase noise and the CFO at the transmit end. The impact of phase noise on OFDM performance mainly lies in two aspects: a common phase error (CPE) and inter-carrier interference (ICI), and impact of the CFO on the OFDM performance mainly lies in the ICI. In an actual system, the ICI has weaker impact on performance than the CPE. Therefore, usually the CPE is preferably compensated for in a phase noise compensation solution.
Phase noise is used as an example. As a frequency band increases, a phase noise level decreases by 20*log(f1/f2). A 2 GHz frequency band and a 28 GHz frequency band are used as examples. A phase noise level of the 28 GHz frequency band is 23 dB higher than that of the 2 GHz frequency band. A higher phase noise level indicates stronger common phase error (CPE) impact and a larger phase error caused by a CPE, as shown in FIG. 1A to FIG. 1C.
Different subcarriers in a same OFDM symbol are under same impact of a CPE. Phase errors on different subcarriers are different because of impact of white Gaussian noise. Therefore, in frequency domain, a plurality of estimated phase noise values are obtained by using a specific quantity of phase noise reference signals, and the plurality of estimated phase noise values are averaged to obtain a CPE, to reduce the impact of the white Gaussian noise to a greatest extent. Theoretically, a larger quantity of phase noise reference signals indicates a better averaging effect and a more accurately estimated CPE. In time domain, because phase noise varies discontinuously, and there is no linear relationship between different symbols, performance is poorer if time domain pilots are sparser. In addition, a larger quantity of phase noise reference signals indicates more occupied time-frequency resources and higher overheads. Therefore, a compromise needs to be made between performance and overheads to determine the quantity of phase noise reference signals.
The prior art provides a phase tracking reference signal (the reference signal may also be referred to as a pilot) design solution, as shown in FIG. 2A-1, FIG. 2A-2, FIG. 2B-1, and FIG. 2B-2. A demodulation reference signal (DMRS) and a phase compensation reference signal (PCRS) (which may also be referred to as a phase tracking reference signal (PTRS), and the PCRS and the PTRS are not uniformly named in the industry currently and are collectively referred to as the PTRS subsequently for ease of description in the present invention) are used to complete channel estimation, phase noise estimation, and data demodulation together for both uplink and downlink. The DMRS is used for channel estimation and data demodulation, and the PTRS is used for tracking a residual phase error. There are a plurality of ports for the DMRS and the PTRS. A same antenna port is used for the PTRS and the DMRS in uplink, and a plurality of ports for the DMRS correspond to a same PTRS port in downlink. In time domain, PTRSs are consecutively mapped, to be specific, a PTRS is mapped to each symbol after the DMRS. In frequency domain, a frequency division manner is used between different ports. A time domain density and a frequency domain density are set to fixed values (an uplink density is 1/96, and a downlink density is 1/48). A quantity of reference signals increases as an effective bandwidth increases. When a data bandwidth is relatively small, there are a relatively small quantity of reference signals, and when the data bandwidth is less than four RBs, no PTRS is mapped, as shown in FIG. 2A-1 and FIG. 2A-2 and FIG. 2B-1 and FIG. 2B-2.
In addition, 2-bit and 1-bit downlink control information (DCI) or uplink control information (UCI) are respectively used for downlink and uplink, to indicate PTRS-related configurations. The downlink is used as an example. The 2-bit DCI is used to indicate whether a base station is to send a PTRS and which port is used if the base station is to send the PTRS. Details are shown in Table 1.
TABLE 12-bit Configuration InformationBitsConfiguration information00Send no PTRS01Send a PTRS by using a port 6010Send a PTRS by using a port 6111Send a PTRS by using a port 60 and a port 61
The prior art has the following disadvantages: PTRSs are consecutive in time domain, and a frequency division manner is used for a plurality of ports in frequency domain. Also, a time domain density and a frequency domain density are fixed values, and a relatively large quantity of subcarriers are occupied and overheads are relatively high when a data bandwidth is large. In addition, the prior art is not flexible because the fixed time domain density and the fixed frequency domain density are used for different scenarios such as different phase noise levels and different moving speeds.